Assembly and characterization of Ru-decorated GaN NWs vertically aligned onto siliconAccording to our previous work, 1D GaN NWs were controllably grown on a 4-inch silicon wafer by employing plasma-assisted molecule beam epitaxy (MBE) technology under N-rich conditions24,25. Ru NPs were subsequently immobilized onto GaN NWs/Si via a simple photo-deposition method (Supplementary Fig. S1 and Experimental Section). As characterized by scanning electron microscope (SEM), the epitaxial GaN NWs decorated with Ru NPs were vertically aligned on the silicon wafer, featuring a length of about 950 nm with an averaged diameter of about 50 nm (Fig. 1A). The well-defined 1D nanostructure renders GaN NWs with high surface area and short charge diffusion length, which is highly favorable for spatially decoupling photons absorption, charges separation, as well as surface chemical reactions. Compared to the pristine GaN NWs (Supplementary Fig. S2), the overall morphology of nanowires is not obviously varied with the decoration of Ru NPs. The high-angle annular dark-field scanning transmission microscope (HAADF-STEM) image illustrates that the Ru NPs with the size of about 19 nm was randomly distributed on GaN NWs surface (Fig. 1B). The lattice spacing of 0.26 nm is assigned to the (002) plane of GaN, in line with XRD patterns (Supplementary Fig. S3), suggesting the c-axis growth direction of the nanowires26,27. The lattice spacing of 0.23 nm is attributed to the (100) plane of metallic Ru28. However, no typical peaks of Ru NPs were observed in XRD patterns, because of its low content (0.11 μmol·cm−2), as characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-OES). The energy dispersive X-ray spectroscopy (EDS) mapping further validated that Ru NPs were successfully assembled on the GaN NWs/Si platform (Fig. 1B).Fig. 1: Structural and physical properties of GaN NWs decorated with/without Ru nanoparticles.A 45°-tilted SEM image of Ru NPs-decorated GaN NWs/Si. HR: H2 activity; TOF: turnover frequency; TON: turnover numbers. B HAADF-STEM and EDS mapping images of Ru NPs-decorated GaN NWs. High-resolution XPS spectra of (C) Ga 2p, (D) N 1 s, (E) Ru 3d. F Bader charge analysis between Ru and GaN. The yellow and cyan regions indicate the gain and loss of electronic charge respectively, with an isosurface of 0.008 e/Å3. Ga, blue; N, yellow; and Ru, salmon. Source data are provided as a Source Data file.X-ray photoelectron spectroscopy (XPS) was carried out to investigate the chemical components of Ru NPs/GaN NWs/Si and the interaction between Ru NPs and GaN NWs (Supplementary Fig. S4). The characteristic peaks of Ga 3d and N 1 s are located at ~20 and ~397 eV, respectively (Figs. 1C, D)29. The high-resolution X-ray photoelectron spectroscopy (HR-XPS) spectra of Ru 3d in Fig. 1E confirmed the presence of metallic Ru species (281.3 eV). The peak located at 282.0 eV was assigned to the Ru-N bond. Further, the observable binding energy shifts of ~0.3 eV and ~0.1 eV in the HR-XPS spectra of Ga 2p and N 1s compared to that of pristine GaN are indicative of the electron redistribution between Ru NPs and GaN NWs. Bader charge analysis was conducted to confirm the above results. As shown in Fig. 1F, a discernible charge transfer is observed from Ru to GaN on the optimized geometry of Ru/GaN with a calculated value of 0.535e. As studied below, such an electronic interaction between Ru and GaN not only stabilizes the nanoparticles against agglomeration but also provides electronic transmission channels, thus catalytically facilitating the ammonia decomposition.As one key component of the photo-thermal-coupling architecture, GaN can generate energetic carriers with sufficient redox potential for ammonia decomposition if excited by appropriate photons as characterized by photoluminescence (PL) spectroscopy (Fig. 2A). The incorporation of Ru species can inhibit the recombination of photoinduced electron-hole pairs30. Time-resolution PL (TR-PL) spectroscopy further revealed that upon decoration with Ru species (Fig. 2B), the average charge lifetime (τavr) of GaN decreased from 2.02 ns to 1.81 ns, suggesting accelerated charge transfer30,31. The above results demonstrate that the decoration of Ru NPs onto GaN NWs is favorable for mediating the charge behavior to catalyze ammonia decomposition. The photo-thermal properties of the architectures were further studied using an infrared thermograph. As shown in Fig. 2C, the surface temperature of Ru NPs/GaN NWs/Si can reach 409.7 °C under 5 W·cm−2, which is relatively higher than that of Ru NPs/Si (Supplementary Fig. S5A). Herein, GaN NWs contribute to the photothermal effect by alleviating the photon scattering due to the well-defined 1D nanostructure32,33. In contrast, when the Si substrate was replaced by sapphire (Supplementary Fig. S5B), the measured temperature dropped to 284.5 °C without varying any other conditions (Fig. 2C). The results show that Si is indeed an ideal platform for assembling a photo-thermal architecture. The well-defined 1D nanostructure of GaN is beneficial to reducing the Rayleigh scattering, thus further increasing the surface temperature of the architecture without changing the illumination. Such a hierarchical architecture does not only provide energetic charge carriers but also enables high localized temperature. It thus holds a grand promise for photo-thermal-coupling ammonia decomposition toward H2, which will be elaborately studied next.Fig. 2: Optical and physical properties of GaN NWs decorated with/without Ru nanoparticles.A PL spectroscopy of GaN NWs/Si decorated with/without Ru NPs recorded with pulse excitation of 80 MHz at a wavelength of 325 nm. B TRPL spectroscopy of GaN NWs decorated with/without Ru NPs by a time-correlated single photon counting technique. τavr: average charge lifetime. C Infrared thermal images of Ru NPs/GaN NWs/Si, GaN NWs/Si, Ru NPs/Si, and Ru NPs/GaN TF/Sapphire surface under 5 W·cm−2 concentrated light illumination. Ru NPs loaded onto commercial GaN thin films on sapphire substrate is defined as Ru NPs/GaN TF/Sapphire. Source data are provided as a Source Data file.Photo-thermal-coupling performance of ammonia decomposition toward H2
The performance of Ru NPs/GaN NWs/Si for ammonia decomposition was tested in a sealed quartz chamber under atmospheric argon. A 300 W Xenon lamp equipped with a quartz lens was used as the light source. A commercial ammonia aqueous solution was used as the feedstock as water is a good medium for NH3 storage and transportation under ambient conditions. Under concentrated light illumination, the aqueous ammonia solution was facilely evaporated due to the strong photo-thermal effect. The critical role of each component of the ternary Ru NPs/GaN NWs/Si architecture was first investigated (Fig. 3A). In the absence of Ru species, the bare GaN NWs/Si platform is almost inactive for light-driven ammonia decomposition although charge carriers and photo-induced heat can be provided under concentrated light illumination at 5 W‧cm−2 (Fig. 3A), suggesting that Ru species are essential for the reaction by serving as active sites. The hydrogen evolution rate was significantly enhanced by the immobilization of Ru NPs (Supplementary Figs. S7 and S8). A maximum value of 1.77 mmol·cm−2·h−1 was achieved at a Ru loading of 0.11 μmol·cm−2, corresponding to an optimal turnover frequency of 16091 h−1, which is an intrinsic metric for evaluating the activity of the catalytic sites (Supplementary Figs. S6 and S7). Herein, the average diameter of Ru NPs was measured to be ~19 nm (Supplementary Fig. S8). A reduced H2 activity of 0.98 mmol·cm−2·h−1 was observed when Ru loading increased up to 0.22 μmol·cm−2 (Supplementary Fig. S6). It is primarily due to the undesired agglomeration of Ru NPs with an average diameter of > 86 nm (Supplementary Fig. S8)34. Despite the high surface temperature arising from the significant photo-thermal effect of silicon, there is almost no hydrogen production over Ru/Si, suggesting the critical role of GaN NWs in offering energetic charge carriers (Supplementary Fig. S9).Fig. 3: On-site H2 production from photo-thermal-coupling ammonia decomposition.A H2 evolution rate over Ru NPs/GaN NWs/Si, GaN NWs/Si, and Ru NPs/Si illuminated by a 300 W-xenon lamp under 5 W·cm−2. B H2 activity over Ru NPs/GaN NWs/Si under dark equipped with an external heating system and concentrated light-illuminating conditions without external heating. The architecture was maintained at the same temperature for better comparison of the performance between photo-thermal-coupling catalysis and pure thermocatalysis. C Arrhenius plots for H2 evolution rate under dark and light conditions over Ru NPs/GaN NWs/Si. D H2 evolution rate over Ru NPs/GaN NWs/Si under 5 W·cm−2 with/without cooling. E H2 evolution rate over Ru NPs/GaN NWs/Si under light irradiation in different spectral ranges (full spectra, ultraviolet, visible, and infrared) with/without external heating source. The temperature of the external heating source is set to 280 °C. F Durability test over Ru NPs/GaN NWs under 4 W·cm−2. Sample, ~0.5 cm2, ~0.36 mg·cm−2; atmospheric argon; 300 W Xenon lamp. Source data are provided as a Source Data file.The light intensity-dependent H2 activity was further measured to study the photo-thermal synergy. As illustrated in Fig. 3B, the H2 evolution rate showed an increasing trend as the light intensity increased, and reached 3.98 mmol·cm−2·h−1 at 5 W·cm−2 with an appreciable turnover frequency (TOF) of 36,182 h−1 without any other energy inputs (Supplementary Fig. S10). Such a great activity is nearly 1-2 orders of magnitude higher than state-of-the-art thermal or photocatalytic ammonia decomposition systems (Table S1). To decouple the photo-excited carriers and photo-induced heat contribution, the catalytic properties of Ru NPs/GaN NWs/Si were both photo-catalytically and thermal-catalytically measured. First of all, light-induced heating was in operando recorded with an infrared thermograph as a function of light intensity (Supplementary Fig. S11). The surface temperature of the as-designed architecture showed an increasing trend as the illumination intensity increased, varying from 274.1 °C at 3.0 W·cm−2 at up to 409.7 °C at 5.0 W·cm−2. Shockingly, the photo-thermal-coupling activity is nearly 1000 times higher than that of pure thermo-catalysis under the same temperatures over the temperature range tested (Fig. 3B). Herein, the photo-thermal-coupling activity was obtained by the only input of concentrated light while the thermo-catalysis was powered by external heating. Of note, as calculated by the Arrhenius equation, the apparent activation energy (Ea) of NH3 decomposition over Ru NPs/GaN NWs/Si is significantly reduced from 1.08 eV to 0.22 eV upon light illumination (Fig. 3C). The above results suggested that the reaction proceeded via photocatalysis, which can be further promoted by the photoinduced thermal effect resulting from the concentrated visible- and infrared light. The influence of light wavelength on the reaction was also studied. As shown in Supplementary Fig. S12, under a measured reaction temperature of 270 °C set by external heating, the introduction of 275 nm under monochromatic light illumination of 29.8 mW·cm−2 in the system results in an enhanced H2 evolution rate of 1.46 μmol·cm−2·h−1 with a high apparent quantum efficiency (AQE) of 1.19% over Ru NPs/GaN NWs/Si. It is much higher than that of the 535 nm under monochromatic light illumination of 285.1 mW·cm−2. Therefore, it is reasonable to speculate that the high-energy photons of <365 nm that can produce energetic charge carriers are critical for superior activity. However, by employing an external cooling system to alleviate the photo-thermal effect of the reaction system (Fig. S13), the measured catalytic activity sharply dropped from 3.98 to 0.06 mmol·cm−2·h−1 (Fig. 3D). Thus, the photo-induced heat also plays an unignored role for promoting ammonia decomposition. Based on the results above, it is rationally hypothesized that for Ru NPs/GaN NWs/Si, upon concentrated sunlight illumination, high-energy ultraviolet is able to produce energetic charge carriers by exciting GaN NWs. Meanwhile, the remaining visible- and infrared light absorbed by the silicon substrate, accounting for a large proposition of the solar energy (~93%), contributed to heating the architecture and offered high-localized surface reaction temperature for ammonia decomposition. Charge carriers and heat work in synergy to promote the reaction by inducing a substantial reduction of activation barriers of ammonia decomposition and increasing the reaction temperature. Such a hypothesis was further validated by wavelength control experiments. The thermal contribution from various regions of the solar spectrum was first studied. As shown in Supplementary Fig. S14, the surface temperature of the architecture was measured by an infrared thermograph. It was found that the full-arc spectrum at 4 W·cm−2 can increase the architecture temperature up to 342.7 °C. By contrast, the surface temperature of the architecture dropped to 100.5 °C if an ultraviolet light filter was employed. Meanwhile, visible and infrared light can heat the architecture to 161.7 and 170.4 °C, respectively. In the absence of an external heat source, the hydrogen activity of Ru NPs/GaN NPs illuminated by ultraviolet light was only about 20% of the full spectrum (Fig. 3E). The incorporation of the infrared and visible light did not show activity for ammonia decomposition toward H2. Notably, once an external heat source was applied for heating the reaction system (280 °C), the introduction of ultraviolet light can greatly improve the activity of H2 up to 1.56 mmol·cm−2·h−1, which is nearly 90% of the activity obtained under the full spectrum. These findings reveal the synergy between charge carriers induced by ultraviolet light and photo-induced heat as a result of visible light and infrared light played a vital role in the performance of Ru NPs/GaN NWs/Si.As a key metric for practical application, the stability of the architecture was also examined (Fig. 3F). A high turnover number (TON) of 3,400,075 moles of hydrogen per mole of Ru NPs was achieved after 400 h of light illumination. Of note, an H2:N2 stoichiometric ratio of 3:1 was observed, suggesting the absence of the undesired side reaction. As characterized by XPS and TEM, there were no significant morphology and chemical component variations for this architecture after the stability test (Supplementary Fig. S15). The aforementioned observations are indicative of the good stability.Temperature-dependent photoluminescence (TD-PL) spectroscopy characterizations were conducted to study the photo-thermal effect on the reaction. It is observed that by increasing the measured temperature from 50 °C to 450 °C, the recombination of photoexcited e−/h+ pairs was evidently inhibited (Fig. 4A)35,36. Hence, the photo-thermal effect does not only promote the reaction by increasing the reaction temperature but also facilitates the separation of photoexcited e−/h+ pairs, which is very beneficial for the reaction. Operando diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was utilized to monitor the key intermediates of ammonia decomposition (Fig. 4B). It is discovered that the peaks at around ~948 cm−1 and ~3255 cm−1 are assigned to the adsorbed *NH3 species1,37,38. The peak intensity at around 1369 cm−1 arising from the *NH2 intermediate increased with the irradiation time39,40, which is well matched with the EPR results (Supplementary Fig. S16). It can be attributed to the continued deprotonation of *NH3 toward *NH2 by the photo-excited holes (NH3 + h+ → NH2 + H+), which had not yet reached steady-state condition even 25 min after since the start because of the saturation of ammonia in the cell. Of note, compared with pure thermal catalysis (Supplementary Fig. S17), the accumulation rate of *NH2 on the surface of the catalyst during the photo-thermal-coupling process is significantly faster (Supplementary Fig. S18), further suggesting that the photo-thermal effect greatly promotes the activation and deprotonation of the reactant molecules. The above results demonstrate the viability of maximally utilizing solar energy to drive NH3 decomposition toward H2 by effectively utilizing photons at various regions (Fig. 4C).Fig. 4: TD-PL spectroscopy and operando DRIFT spectra.A TD-PL spectroscopy of Ru NPs/GaN NWs/Si. B Operando DRIFT spectra of ammonia decomposition over Ru NPs/GaN NWs/Si under light illumination of 4 W·cm−2. C Schematic diagram of the synergy between charge carriers and photo-induced heat for promoting ammonia decomposition over Ru NPs/GaN NWs/Si. Source data are provided as a Source Data file.Possible mechanism of ammonia decompositionThe electronic state of the catalytic interface can affect the reaction significantly. Thereby, the redistribution of photo-induced electrons at the interface between GaN and Ru was characterized by in situ irradiated XPS (ISI-XPS) (Fig. 5A). It is clear that under light illumination, the binding energy of Ru 3d illustrated a slightly negative shift. Conversely, a marked positive shift in the HR-XPS spectra of Ga 2p and N 1s was observed (Supplementary Fig. S19). Herein, these observations validated that Ru NPs behave as effective electron sinks as reported41,42. The electron redistribution from GaN to Ru under light illumination is critical for superior activity41. NH3 temperature-programmed desorption (NH3-TPD) characterizations were conducted to investigate the adsorption behavior of ammonia molecules. When GaN was decorated with Ru NPs, the desorption signal of NH3 was evidently enhanced (Fig. 5B). It is indicative that the incorporation of Ru species can promote the adsorption of NH3 molecules onto the GaN NWs surface, thus favoring hydrogen evolution from ammonia decomposition. Operando DRIFT reveals the evolution track of ammonia at the molecular level (Fig. 4B and Supplementary Fig. S20). Interestingly, the accumulation rate of *NH2 intermediates on the pristine GaN is much lower than that of Ru NPs/GaN NWs/Si (Fig. 5C), suggesting that the absence of Ru stabilizes the *NH2 for further dehydrogenation, which will be discussed by DFT calculations next. Isotope experiments were conducted in NH3/D2O medium to elucidate the origin of hydrogen. It is found that H2 almost originated from NH3 (Supplementary Fig. S21). Moreover, there was not an evident variation in reaction kinetics between NH3/H2O and NH3/D2O, which is indicative of no kinetic isotope effect (KIE) (Supplementary Fig. S22). The ratio of H2 and N2 was detected online with the use of a reaction chamber linking the gas chromatography (GC) to prevent the interference of N2 in the air. GC analysis of the product suggested a stoichiometric H2/N2 ratio of 1/3 (Supplementary Fig. S23), with no other byproducts aside from H2 and N2. Based on the above results, it is reasonably speculated that water does not actively participate in the reaction; rather, it serves primarily as an ideal storage and transportation medium of NH3 under ambient conditions, which is important for practical applications.Fig. 5: Spectral characterization and DFT calculation.A ISI-XPS spectra of Ru 3d with or without light illumination. B NH3-TPD spectra of bare GaN NWs and Ru NPs/GaN NWs. C Calculated slope change of the peak intensity of *NH2 based on operando DRIFT spectra in Fig. 4B and Fig. S20. D LDOS for pristine GaN and Ru/GaN, respectively. The black dashed line indicates the position of the Fermi level. E The calculated free energy ∆G diagrams for NH3 decomposition on GaN and Ru/GaN. The values in the figures indicate the energy difference for the potential-limiting step of the reaction. F Schematic diagram of ammonia decomposition process over the Ru/GaN interface. Source data are provided as a Source Data file.DFT calculations were performed to gain insights into the reaction mechanism at the molecular level. Firstly, three optimal surface models of Ru (0001), GaN (10\(\bar{1}\)0), and Ru/GaN were constructed (Supplementary Fig. S24). The electronic properties were analyzed by plotting the local density of states (LDOS) for GaN and Ru (Fig. 5D). Pristine GaN with a large bandgap (3.4 eV) is generally not conducive to electron transfer in the reaction43. However, after decorating with Ru NPs, the metal states appeared near the Fermi level of GaN, enabling a high conductivity of this architecture. More importantly, the strong interaction between GaN and Ru arising from the electron redistribution formed a new state around the Fermi level for Ga and N atoms at the interface, thus facilitating the photoexcited electron transfer from GaN to Ru and its subsequent participation in the ammonia decomposition. This computational result was in well consistency with the ISI-XPS characterizations (Fig. 5A). Furthermore, the ammonia decomposition pathway on the three surfaces was calculated and the results are shown in the Gibbs free energy diagram (Fig. 5E). Upon the initial adsorption stage, the NH3 molecule was strongly attached to the GaN surface, with the N atom binding to the Ga atom, and then a H atom was captured by the near N atom in GaN (Supplementary Fig. S25). Of note, the NH3 adsorption energy over the GaN is slightly reduced from −0.98 eV to −1.15 eV after the immobilization of Ru NPs (Supplementary Fig. S26 and Table S2), indicating an enhanced adsorption capacity of Ru/GaN for NH3 molecule. It is well matched with the NH3-TPD measurements (Fig. 5B). Following the step of *NH3 → *NH2, *NH2 intermediate was stabilized onto the catalytic interface for the subsequent decomposition process. Particularly, in the case of GaN, we observed that the potential-determining step (PDS) exhibited high endergonicity with a significant energy difference of 1.78 eV. In contrast, the presence of a Ru cluster on the GaN surface stabilized the *NH2 intermediates and thus significantly reduced the barriers and shifted the PDS to the desorption of the second H atom, resulting in a smaller free energy change of only 0.58 eV (Fig. 5E and Supplementary Fig. S27). Based on the ISI-XPS and LDOS results (Fig. 5A and Fig. 5D), the transfer of photoinduced electrons enables the electron-rich Ru sites to a lower ∆GH value, which is a favor for the accumulation of *H for the final formation of H2 (Fig. 5F). What’s more, the adsorption energy of water molecule is much higher than that of NH3 molecule over the Ru NPs-decorated GaN surface (Supplementary Figs. S26, S28, and S29), and significantly increase the energy barrier for H2O dissociation on both GaN and Ru/GaN surfaces compared to NH3 decomposition (Supplementary Fig. S30). Therefore, in this study, water primarily functions as a medium of ammonia rather than a hydrogen source, in line with the isotopic experimental results.On-site hydrogen evolution under natural concentrated sunlight illuminationTo assess the practical viability, the as-assembled architecture was tested under natural concentrated sunlight. The homemade experimental setup mainly consisted of a Fresnel lens, support frame, and quartz reaction chamber (Fig. 6A). The natural sunlight was concentrated by a cheap and simple lens, which is beneficial for improving the performance and reducing the usage of the catalyst. As depicted in Fig. 6B and Supplementary Fig. S31, the H2 evolution rate is directly related to the natural sunlight illumination conditions, which is varied from 0.07 mmol·cm−2·h−1 to 0.17 mmol·cm−2·h−1, as a result of the varied light intensity. This observation suggests that the H2 rate over the architecture is highly dependent on the light intensity. What’s more, the solar-to-hydrogen (STH) efficiency was also calculated, reaching an optimal value of 5% from 13:00 pm to 14:00 pm (Supplementary Fig. S32). After 14 h of operation, San illustrious TON of 51,689 moles H2 per moles Ru NPs was achieved under naturally concentrated sunlight conditions (Fig. 6C). Such an outdoor test revealed the viability of utilizing natural sunlight for hydrogen production from ammonia. To step forward to practical application, as the priority, the fabrication cost of the architecture needs to be significantly reduced. Meanwhile, long-term stability is required for the architecture.Fig. 6: Outdoor experiments.A Image of outdoor test setup equipped with Ru/GaN NWs/Si. B Activity and (C) TON of Ru NPs/GaN NWs for ammonia decomposition under concentrated natural sunlight without external heating, the inset in (B) is the digital picture of Ru NPs/GaN NWs/Si. Source data are provided as a Source Data file.
Utilizing full-spectrum sunlight for ammonia decomposition to hydrogen over GaN nanowires-supported Ru nanoparticles on silicon
